1. Field of the Invention
This invention relates broadly to hydrocarbon processing and more specifically to the alkylation of saturated and unsaturated aliphatic hydrocarbons. The invention is directly concerned with improving the efficiency of processing olefinic and paraffinic C.sub.3 to C.sub.5 hydrocarbons for the recovery of high value hydrocarbon products by the fractionation, selective hydrogenation and alkylation of such feed components.
2. Prior Art Information Statement
The production of motor fuel by the alkylation of light paraffins with C.sub.3 and/or C.sub.4 olefins is a widely practiced commercial process. Liquid phase hydrofluoric acid (HF) is often employed as the catalyst. This process is described in U.S. Pat. Nos. 3,073,878; 3,080,438; 3,249,650; 3,515,770; 3,560,587; 3,686,354; 3,867,473; 3,925,502, 4,139,573 and 4,161,497. The process is also described in the article starting at page 78 of the Feb. 11, 1974 issue of The Oil and Gas Journal. These references describe process conditions, process equipment, the regeneration of the HF, and fractionation and treating procedures required in the process.
U.S. Pat. No. 3,655,621 issued to A. S. Kasperik et al. illustrates a process for the selective hydrogenation of C.sub.4 diolefins in an alkylation feed stream employing a catalyst comprising presulfided nickel supported on a refractory base. In U.S. Pat. No. 3,234,298 issued to W. C. van Zijll Langhout et al., a process is disclosed for the selective hydrogenation of light, diene-containing cracked hydrocarbon oils. This process is employed to increase the stability of such materials as pyrolysis gasoline and kerosene obtained by severe thermal cracking operations. Such hydrogenation is desirable to reduce the gum-forming characteristics and other undesirable properties of these hydrocarbon mixtures. The process is described as being applicable to diene-containing hydrocarbons ranging from C.sub.3 to C.sub.18 in carbon number. The process employs a catalyst comprising sulfided nickel on alumina or sulfided molybdenum on alumina.
It is also known from U.S. Pat. No. 3,696,160 issued to K. D. Chomyn that it may be beneficial to selectively hydrogenate diolefins to monoolefins in certain hydrocarbon streams. This reference is directed to the selective conversion of propadiene and butadiene contaminants in propylene and butene charge stocks employed in alkylation processes for the production of aviation and motor fuel. In this alkylation process, a C.sub.3 -C.sub.4 feed stream is converted to a high octane C.sub.7 -C.sub.8 product. It is stated that a small diolefin content in the alkylation feed stream is undesirable because of increased acid consumption as a result of forming tarry acid-diolefin condensation products, which decreases the profitability of the process. The reference indicates that supported nickel and palladium catalysts are excellent hydrogenation catalysts in the diolefin conversion service, but that their tendency to deactivate in sulfur-containing feedstocks limits their utilization. The reference also discloses the use of a sulfided nickel-tungsten catalyst.
When combining a selective hydrogenation process with an HF alkylation process, it is necessary to remove light gases such as ethane, methane, and hydrogen from the hydrogenation unit effluent before it is charged to the alkylation unit. Otherwise the light ends will require venting of the HF alkylation unit with resulting HF acid losses.
In conventional flow schemes for butene alkylation, feed is derived from a depropanizer column that is also used to remove propane from an alkylation zone recycle stream. This depropanizer could be utilized to remove light gases but this would require further processing to produce a light ends free propane-propylene. In addition, a drawback to the conventional flow scheme is that to avoid fluoride contamination of the propane-propylene fraction, the entire alkylation zone recycle stream is treated to remove fluorides.